Ultrasonic transducers are devices that radiate and receive sound waves in or above the audible range (approximately 20 Hz to 20 kHz), and are widely used for medical purposes, in non-destructive testing, etc. Piezoelectric devices, a typical example being PZT (Lead Zirconate Titanate), are presently most widely used as ultrasonic transducers. However, in recent years, the development of ultrasonic devices called Capacitive Micro-machined Ultrasonic Transducers (hereinafter, CMUTs), which utilize an operation principle that differs from piezoelectric types, has advanced, and is beginning to be put to practical use. CMUTs are fabricated by applying semiconductor techniques. They are ordinarily formed by burying an electrode material in a substrate (or the substrate itself may sometimes serve as an electrode) made of a material that is used in semiconductor processes, e.g., silicon, etc., and by securing a fine (e.g., 50 μm) and thin (e.g., several μm) diaphragm with supporting walls around the diaphragm, etc. A cavity is provided between the diaphragm and the substrate to allow the diaphragm to vibrate. An electrode material is buried within this diaphragm as well. By thus having independent electrodes disposed in the substrate and the diaphragm, the substrate and the diaphragm function as a capacitance (capacitor). By applying a voltage across both electrodes (a bias voltage is ordinarily applied in advance), they function as an ultrasonic transducer. When an AC voltage is applied across both electrodes, the electrostatic force between the electrodes varies, causing the diaphragm to vibrate. If, at this point, there is some medium that is in contact with the diaphragm, the vibration of the diaphragm will propagate within the medium as a sound wave. In other words, it is possible to radiate sound. Conversely, if a sound wave is transmitted to the diaphragm, the diaphragm will vibrate in accordance therewith, and as the distance between both electrodes varies, an electric current will flow between both electrodes, or the voltage across both electrodes will vary. By extracting an electric signal of this electric current, voltage, etc., it is possible to receive sound waves.
Important indicators that determine the performance of an ultrasonic transducer include the acoustic pressure transmitted and receive sensitivity. To increase acoustic pressure and receive sensitivity, the greater the area that vibrates, the better. The area that vibrates is dependent on the shape of the diaphragm. In the case of a circular, square or regular hexagonal diaphragm, since the diaphragm is secured from around in a generally uniform manner, the diaphragm is only able to vibrate near its center. As a result, in effect, only approximately 30 to 40% of the cavity area is used effectively. On the other hand, in the case of an elongate rectangular (oblong) diaphragm, the extent to which it is bound from around is mitigated, and displacement in a more even manner becomes possible as compared to a circular diaphragm, etc. In this case, approximately 60% of the area vibrates effectively. Thus, from the standpoint of improving acoustic pressure and receive sensitivity, an elongate rectangular shape is preferable. However, when a shape that is elongate to some extent is adopted, as in a rectangular diaphragm, characteristic high-order vibration modes occur. The various vibration modes that occur in the diaphragm have an influence on acoustic characteristics, e.g., radiated acoustic pressure, frequency characteristics, pulse characteristics. Accordingly, controlling vibration modes becomes extremely important.